1. Field of Invention:
This invention relates generally to offset presses in which paper to be printed is fed into a nip between a blanket cylinder and an impression cylinder, and more particularly to an adjustable axle-bearing assembly for the impression cylinder which makes it possible to set the nip to establish a printing pressure appropriate to the paper being printed.
2. Status of Prior Art:
The planographic printing process is based on an 18th Century invention of Alois Seefelder, which makes use of a smooth stone and exploits the antipathy of grease and water, the printing ink adhering only to greasy image areas on the stone face. Planographic printing is also called lithography, "litho" being the Greek term for stone. Except as an art medium, some lithography has ceased to exist.
Modern offset printing is an extension of lithography and uses a thin metal relief image plate, the inked image being transferred from a plate cylinder to a rubber-covered cylinder called the blanket cylinder. From the blanket cylinder the image is "offset" onto a paper sheet carried by an impression cylinder, the paper being fed into a nip between the blanket and impression cylinders. Rotary offset presses make possible increased production speeds, improved quality in the reproduction of fine tones, and a substantial reduction in the number of impressions required to reproduce full color copy.
The principal raw material for paper making is wood pulp, but better papers are made with cotton linters or rags. Papers are often calendered, coated or laminated and vary not only in grade or thickness, but have different finishes. Hence, a printing pressure appropriate to one type of paper may result in poor quality printing in another type of paper of the same thickness.
The printing pressure applied to the paper fed into the nip between the rubber-covered blanket cylinder and the impression cylinder depends on the size of the nip therebetween. In the typical offset press, the nip size can be adjusted to a limited degree. Since the quality of printing depends on the printing pressure, with some papers even the maximum degree of nip adjustment available on the typical press on the pressure is insufficient to afford the desired quality of printing.
The need exists, therefore, for means to adjust the size of the nip between the impression cylinder and the blanket cylinder to provide a range of printing pressures appropriate to the full range of papers that may be fed into this nip.